Image engine having multiple image sources with different response times for color operation

ABSTRACT

A full color image projection system is provided using two non-color-specific image sources and color-specific filters. The system is capable of projecting an image using one primary color from one image source and the other two primary colors from another image source. The system uses slower speed image sources than would be required with one source alternating between three colors, and exhibits higher resolution than would be obtained from a color-specific image source.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to projection displays, and moreparticularly to a technique for faster color display from liquid crystaldisplay (LCD)-generated images

[0003] 2. Description of the Related Art

[0004] With the ever increasing graphical nature of computer userinterfaces, improving image display devices by improving the size andquality of displayed images generated by digital and analog signals isimportant. The two most popular image display devices are the cathoderay tube (CRT) and the LCD. Although large screen CRTs are available,they often are bulky. Slimmer screens can be made using various imagegenerating devices, such as the LCD, polymer dispersed liquid crystaldisplays (PDLCDs), or other LCD technology, but, at present, screen sizeis limited by manufacturing considerations.

[0005] Projection of an image created by digital or analog signals,rather than by direct imaging, can be an efficient and economical way toincrease display size, provided the overall system size is practical andimage quality is acceptable. Projection of color images, however,presents some problems associated with speed limitations of theparticular device used to convert the image signal to an image that canbe displayed.

[0006] In color displays, there are three major systems for producingdifferent colors and color brightness. One system uses color-specificpixels, in which each pixel transmits only one of the red, green, orblue components needed for a full color image. In this system, thepixels are arranged in groups of red, green, and blue. A particularcolor is achieved in an area of the pixels by turning “on” or “off” theappropriate pixels in that area. For example, if purple is the desiredcolor, the green pixels in the area would remain off while the red andblue pixels would be turned on. Displayed image brightness may also becontrolled by turning the pixels on and off. If bright purple isdesired, for example, the red and blue pixels in the area would remainon for longer periods of time than for a less bright purple. The red andblue pixels are turned on and off at a higher rate for the bright purplethan for the less bright purple. The greater the percentage of “on”time, the brighter the color.

[0007] Another color display system is similar to the system describedabove in that each pixel transmits only one of the primary colors. Inthis second system, the pixels are arranged in groups of red, green, andblue. To achieve a particular color in a pixel area, appropriate pixelsin the area are turned on or off. Brightness is controlled by varyingthe amount of light transmitted by an “on” pixel, rather than by turningoff some of the pixels. This system provides better resolution than asystem that leaves pixels unilluminated to achieve shades of displayedcolor.

[0008] As will be appreciated, systems that rely on color-specificpixels may significantly diminish image resolution compared to systemsthat employ any of their pixels to create an image at any one time,irrespective of color requirements. Moreover, systems that aresubpixelated, color-specific, and that have a limited subpixel size mayexhibit diminished image resolution compared to systems that havesmaller subpixel size. At some point, however, reducing subpixel sizemay be prohibitive in terms of cost. Cost may also be a problem forsystems that use three imagers (e.g., LCDs), one each for red, green,and blue light, and a dispersing element, such as a prism, to separatethe colored light from white.

[0009] One technique that avoids both subpixelation and the use of threeimagers in colored displays is known as field sequential color. Fieldsequential color systems comprise the other major system for producingdifferent colors and color brightness. Each pixel transmits or reflectsred, green, and blue light sequentially in time. When the sequence istransmitted sufficiently fast, the human brain integrates and perceivesthe three light colors as a single blended color, determined, to acertain extent, by the relative proportion of the color inputs. If thetransmission is not fast enough, however, the image may appear smearedand the color integration incomplete, causing so-called “rainboweffects.” To reduce such effects, the three colors must be sequenced ata relatively high rate within a video frame. For example, with a framerate of approximately 60 to 200 Hz, the corresponding three-color(subframe) change rate must be approximately three times this range, or180 to 600 Hz.

[0010] In sequential color displays, color hue and brightness areusually controlled in the time domain. This arises because most LCDs orother imagers capable of the necessary speeds are bi-stable devices, notanalog. Digital devices provide only fully-on and fully-off periods,while analog devices can vary the intensity substantially continuouslybetween the fully-on and fully-off states. For example, with a digitaldevice, if a bluish-purple hue is desired from a certain pixel for atime period, the pixel is electronically controlled to transmit bluelight longer than red light, and to transmit no green light during thattime period. Pulse width modulation may be used to provide suchelectronic control. Adding pulse width modulation to the requirement forhigh speed sequencing may, however, present a practical limitation,because the LCD must be capable of very high on-off switching rates. Asan example, assume that 24-bit color is provided, i.e., 8-bits or 256color levels or values each for red, green, and blue. If analog LCDscould be used, then at approximately 300 to 600 Hz, a different analogvoltage level would have to be applied to each pixel, each voltageassuming one of 256 values corresponding to the 256 color values.Simultaneously, a color filter would have to be switched to pass lightcorresponding only to the color to be displayed. The problem, as notedabove, is that most analog displays are too slow for this type ofsystem.

[0011] Some ferroelectric LCDs (or FLCDs), on the other hand, arebi-stable devices, and are capable of very high switching rates. Theycan be used to provide the equivalent of analog color levels via pulsewidth modulation switching in the time domain. For example, each 300thto 600th of a second subframe interval may be further divided into 6-bittime slots, or 64 time divisions using pulse width modulation. Todisplay at {fraction (1/64)}th color intensity with a filter, such as ablue filter, the FLCD would be turned on for {fraction (1/64)}th of the300th to 600th of a second subfame that the blue filter is engaged.Although 6-bits per color, or a total of 18-bits for three colors, caneffectively be displayed with a single currently available FLCD, 24-bitcolor cannot be reliably displayed. As a result, trade-offs must bemade, either in frame rate, color separation, or color capability.Additional information about the technical difficulties involved inusing FLCDs in field sequential color systems may be found in atechnical disclosure entitled “FLC/VLSI Display Technology,” dated Dec.1, 1995, published by Displaytech, Inc., which is incorporated byreference herein in its entirety.

[0012] It is noteworthy that Texas Instruments has developed a so-called“one-color plus two-color” field sequential color system that usesdigital or deformable micromirror devices (DMDs) capable of 24-bit coloras imagers. The Texas Instruments system, however, uses two DMDs foraddressing lamp spectrum issues, while not addressing imaging speed orbandwidth issues.

[0013] The present invention is directed to providing a full color,image producing system that uses available FLCDs, while avoiding some ofthe aforementioned design trade-offs.

SUMMARY OF THE INVENTION

[0014] In general, in one aspect, embodiments of the invention feature asystem used in image projection. The system includes a first filter forreceiving light, for directing a first portion of light to a first imagesource for first portion operation, and for passing a second portion ofthe light to a second image source for second portion operation. Thesystem also includes an electronic color switch for alternately allowingcomponents of the second portion to be passed to the second imagesource.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Other objects and advantages of the invention will becomeapparent upon reading the following detailed description and uponreference to the drawings in which:

[0016]FIG. 1 is a plan view of components of a projection display systemin accordance with an embodiment of the invention;

[0017]FIG. 2 shows an implementation of components of the projectiondisplay system in FIG. 1 in accordance with an embodiment of theinvention;

[0018]FIG. 3 is a plan view of components of a projection display systemin accordance with an embodiment of the invention;

[0019]FIG. 4 is a graph illustrating color-selectivity of a component inFIG. 3;

[0020] FIGS. 5A-5D show views and states of an exemplary implementationof the components in FIG. 3 in accordance with another exemplaryembodiment of the invention;

[0021]FIGS. 6A and 6B show views and states of an implementation of thecomponent in FIG. 3 in accordance with another exemplary embodiment ofthe invention;

[0022]FIGS. 7 and 8 are side views of display systems in accordance withembodiments of the invention; and

[0023]FIG. 9 is a view of a portion of the display system in FIG. 8.

[0024] While the invention is susceptible to various modifications andalternative forms, exemplary embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of exemplaryembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

[0025] Illustrative embodiments of the invention are described below. Inthe interest of clarity, not all features of an actual implementationare described in this specification. It will of course be appreciatedthat in the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

[0026] Referring now to the drawings, FIG. 1 illustrates a highresolution color image system 10 in accordance with an embodiment of theinvention. The system 10 can be a reduced cost system. A light source 12provides a portion of light 13 reflected by both a reflecting polarizer18 and a one-color or color-selective filter 20 to a one-color imager orimage source 14 (e.g., an LCD) as light 15. The light source 12 providesan additional portion of the light 13 reflected by the reflectingpolarizer 18 and transmitted through both the one-color filter 20 and acolor switch or color-operative filter 22 (e.g., an electro-optic colorswitch) to a two-color imager or image source 16 (e.g., an LCD) as light23. The LCD 14 operates to provide one color, for example, one of theprimary colors, for a colored image. The other LCD 16 operates toprovide remaining colors, for example, the remaining two primary colors,for the colored image.

[0027] The light source 12 may be a highly efficient source of light,such as the light sources disclosed in U.S. pat. application Ser. No.08/747,190, filed Nov. 12, 1996, entitled “High Efficiency LampApparatus for Producing a Beam Polarized Light,” which is incorporatedby reference herein in its entirety. The light source 12 may also be ahigh intensity discharge (HID) lamp, such as the lamps in U.S. Pat. No.5,404,076, entitled “Lamp Including Sulfur,” and U.S. Pat. No.5,606,220, entitled “Visible Lamp Including Selenium or Sulfur,” bothissued to Dolan et al., and in PCT Pat. application No. PCT/US97/10490,International Publication No. W097/45,858, entitled “Multiple ReflectionElectrodeless Lamp With Sulfur or Selenium Fill and Method for ProvidingRadiation Using Such a Lamp,” by MacLennan et al., which is alsoincorporated by reference herein in its entirety. The light source 12may be a white light, “quasi”-white light, primary color light, or otherthree-color light source. Quasi-white light is produced when there is adeficiency in one or more color components relative to other componentsin the light. If the light source 12 has a weak spectral intensity inone portion of its color output spectrum, for example, in the red,choosing that weak portion of the color spectrum for imaging with theone-color LCD 14 can compensate for the weakness, as will be describedbelow. Any one color, however, such as one of the primary colors, can bechosen to be the light 15 for the LCD 14. The light 23 for the LCD 16can then be composed of the other two primary colors. For purposes ofillustration and ease of description only, the color labels “R” for red,“G” for green, and “B” for blue are included in FIG. 1 for the incidentand reflected light in the color image system 10. Moreover, theone-color LCD 14 and the two-color LCD 16 are described as the “red” LCDand the “green-blue” LCD, respectively.

[0028] The reflecting polarizer 18 may be constructed of areflecting/polarizing material that does not substantially absorb light.The reflecting polarizer 18 is a reflecting linear polarizer as opposedto an absorptive linear polarizer. The reflecting polarizer 18 may bemounted in the optical path of the light source 12 and both the one- andtwo-color LCDs 14 and 16. The reflecting polarizer 18 has an intrinsicoptical axis or direction of orientation that, when orientedappropriately, enables it to be used for separating polarizationcomponents in the incident light 13 by substantially reflecting only onepolarization component of the light 13 while substantially transmittingits orthogonal polarization component. For example, “S”-polarized lightmay be reflected and “P”-polarized light transmitted. The reflectingpolarizer 18, because it is substantially reflective and nonabsorptive,can handle high power light sources, such as the light source 12,without absorbing significant thermal energy.

[0029] In certain embodiments of the invention, a reflector, such asreflector 28 shown in FIG. 1, is also substantially nonaborptive, and,therefore, can handle high power light sources. The reflector 28 may beused when the light source 12 is capable of accepting back light of thepolarization and/or colors that transmit through rather than reflectfrom the reflecting polarizer 18. This “unwanted” polarization (and/orcolors) is reflected from the reflector 28 and transmitted again throughthe reflecting polarizer 18 back to the light source 12. Even inembodiments that include or do not include the reflector 28, it stillmay be possible for the unwanted polarization and/or colors of light tobe reflected back to the light source 12 by the reflecting polarizer 18after passing through other portions of the system 10, as will beappreciated by those skilled in the art. The light accepted back by thelight source 12 may advantageously be used to “optically pump” lightsources of certain types that could be used for the light source 12.Examples of such light sources are disclosed in the aforementioned U.S.pat. application Ser. No. 08/747,190 and in prior, co-owned U.S. pat.application Ser. No. 08/771,326, filed Dec. 20, 1996, entitled“Polarized Light Producing Lamp Apparatus That Uses Low TemperaturePolarizing Film,” which is incorporated by reference herein in itsentirety.

[0030] The reflecting polarizer 18 may be constructed of any materialthat passes light of a desired linear polarization and reflectssubstantially all light of other polarizations, including unpolarizedlight. The reflector 28 may be similarly constructed. The reflectingpolarizer 18 (and the reflector 28) may also be a relativelynon-angle-specific reflecting polarizer. One such reflecting polarizermaterial is double brightness enhancement film (DBEF), a variety ofmultilayer optical film (MOF), which is commercially available fromMinnesota Mining & Manufacturing Company. MOF can be used for thereflecting polarizer 18 and the reflector 28 and thereby avoid using anabsorptive polarizer or material. Such nonabsorptive materials alsoafford an advantage in terms of thermal energy dissipation. For example,high power light of the polarization that is reflected back to the lightsource 12 by the reflector 28 (or the reflecting polarizer 18) willlikely not heat up the system 10, which could otherwise exhibitdetrimental thermal effects if that light were not reflected. Thus, anabsorptive material or polarizer is not desirable, and the reflectingmaterial or polarizer can replace such absorptive materials orpolarizers to avoid or reduce these thermal effects. Nevertheless, anabsorptive material could be used for the reflector 28 in certain otherembodiments, but there would be no or little optical pumping due to anyreflections from such an absorptive material.

[0031] As illustrated in FIG. 1, linearly polarized red, green, and blue(RGB) light 13 is reflected from the reflecting polarizer 18 toward theone- and two-color LCDs 14 and 16. The reflected light first reaches theone-color filter 20, which may be a dichroic filter or a dichroic mirrorthat reflects certain colors of light as the light 15, while allowingother colors to pass through as the light 23. The one-color filter 20 islocated between the reflecting polarizer 18 and the one- and two-colorLCDs 14 and 16. For purposes of illustration and ease of descriptiononly, it is assumed that the one-color filter 20 is a red filter andthat the light 15 is a red light component of the RGB incident light 13,now polarized by the reflecting polarizer 18. The light 15 reflected bythe one-color filter 20 is transmitted to the red LCD 14, while the tworemaining components, for example, the green and blue light components,are permitted to pass through the one-color filter 20, and istransmitted to the green-blue LCD 16 as the light 23. In otherembodiments, the one-color filter 20 could reflect the green or bluelight components, or other components, and allow passage of remainingcomponents.

[0032] For controlling the light 23 incident on the two-color LCD 16, adevice, shown in FIG. 1 as the color switch 22, sequentially andalternately switches between the two remaining color components in thelight 23. The color switch 22 may be a color filter (e.g., a two-colorfilter), switch, or light valve. The switch 22 can be electronically ormechanically controlled to alternately pass one primary color, and thenthe other primary color in the light 23, while reflecting back the coloror colors not passed. The color switch 22 is located in the optical pathof the two-color LCD 16, but outside the optical path of the one-colorLCD 14. With this arrangement, the two-color LCD 16 imparts alternatingcolor images on the light 23 that alternate between two colors, and theone-color LCD 14 imparts a one-color corresponding image on the light15. Thus, the LCD 16 provides two color sequential components and theLCD 14 provides the remaining component for the displayed image to adisplay screen 24 as the light 25 (e.g., as might be useful in a fieldsequential color system, as will be appreciated by those skilled in theart). As can also be appreciated, any type of mechanical or electronicdevice capable of sequentially and alternately filtering out one of thetwo colors in the light 23 and passing the other color at switch(subframe) rates of approximately 200 to 400 Hz may be used for thecolor switch 22. This color switch rate is less than the approximate 300to 600 Hz rate for a three-color/single imager system. Moreover, theintensities of the two colors may be lower than the intensity that wouldbe required for them for the same integrated color brightness in thethree-color/single imager system. This is because the two colors have alower color switch rate (e.g., approximately {fraction (2/3)} thevalue), and can be used for imaging for a longer period of time (e.g.,approximately {fraction (3/2)} times longer), leading to the lowerintensities (e.g., approximately {fraction (2/3)} the values). The third(continuous) color component going to the one-color LCD 14 could remainat the same intensity needed for the same color brightness in thethree-color/single imager system.

[0033] Further, in accordance with an embodiment of the invention, theone-color LCD 14 is chosen to be sufficiently fast for one-colorcontinuous display and the two-color LCD 16 is chosen to be sufficientlyfast for two-color sequential display. However, the system 10 is notsufficiently fast for three-color sequential display at color switchrates of 300 to 600 Hz. Color sequential filtering with the two-colorLCD 16 and the switch 22, and continuous color filtering with theone-color LCD 14, enables the use of slower speed LCDs than would berequired for a three-color/single LCD imager system using thethree-color field sequential technique. Moreover, such an implementationmay not be as expensive or bulky as a three imager system.

[0034] Referring again to FIG. 1, in operation, a controller orcontroller device 26 (e.g., a programmable controller) provides controlvoltage signals simultaneously to arrays of pixels (not shown), whichare individually electrically addressable, in the red LCD 14 and thegreenblue LCD 16. The controller 26 may be a video controller that canaccept as input raw color or encoded video data for use in controllingthe LCDs 14 and 16 for imaging. Each of these pixels in the LCDs 14 and16 have different electrical states that either reflect the incominglight with little or no polarization shift or reflectpolarization-shifted light (e.g., with an approximately 90° polarizationchange) to create colored image components. The LCDs on 14 and 16 thusreflect or transmit light that contains the same image, albeit differentcolor components of that image, to the display 24 as the light 25. Thelight that is not polarization-shifted reflects off the reflectingpolarizer 18 back toward the light source 12, and the pixelscorresponding to this light appear dark in the image on the display 24.The pixels that reflect the polarization-shifted light, however, appearbright. The phase shift occurs because the LCDs 14 and 16 each comprisequarter-wave phase retarders in one of the states and the light 15 and23 make double passes through each retarder. In the illustrated example,the red image imparted on the light 15 and reflected from the red LCD 14is reflected by the one-color red filter 20 back through the reflectingpolarizer 18 to the display screen 24. The alternating green and blueimages imparted on the light 23 and reflected from the green-blue LCD 16pass through the one-color red filter 20 and the reflecting polarizer 18(except for the light not polarization-shifted) to the display screen24. In this way, the red component and the green and blue components ofthe colored image are merged as the light 25 to form a full color imageon the screen 24.

[0035] It can be shown how currently available FLCDs, having a bandwidthof approximately 100 KHz, may be used for the LCDs 14 and 16. First,consider a single imager being used for 8-bits per color (i.e., 8-bitcolor depth) operation. Assume that a 200 Hz frame rate is to be usedinstead of 100 Hz. This is a valid assumption because it has been foundthat a 200 Hz frame rate is a reasonable minimum, for example, in fieldsequential color systems, that may be used to substantially avoid therainbow effects mentioned above. Therefore, a bandwidth of at leastapproximately 200 Hz (frames/sec)×3 (colors/frame)×256 (colorvalues/color=color depth=8 bits/color)=153.6 KHz (color values/sec)would be required. This calculated value is greater than the 100 KHzbandwidth. Moreover, there may be dead time between colors. Thus, 8-bitsper color (or 24-bit full three-color) operation may not be possiblewith a single FLCD imager, while also substantially avoiding the rainboweffects. Operation within the bandwidth limitations of FLCDs may,however, be possible using 6-bit color depth with the LCD 14 being aone-color imager and the LCD 16 being a two-color imager. The LCD 14will have little problem operating within the 100 KHz bandwidth, becausethe one-color (e.g., red) may be continuously provided. On the otherhand, for the LCD 16, if operation occurs using a color depth of 6bits/color, then the bandwidth requirement would be approximately 200 Hz(frames/sec)×3 (colors/frame)×64 (color values/color=color depth=6bits/color)=38.4 KHz (color values/sec), a value well within the 100 KHzlimit. A 180 Hz frame rate, which has been used in some systems, willalso exceed the 100 KHz limit. Therefore, the present invention willallow a combined full three-color operation without the need to bedriven beyond the capability of currently available FLCDs, albeit usinga combination of the one-color and the two-color imagers. In otherwords, the response times of both imagers or LCDs 14 and 16 (i.e., inthe present invention) are too slow for 8-bit color depth, three-coloroperation, but not too slow for 6-bit color depth, three-coloroperation.

[0036] Referring again to FIG. 1, the controller 26 supplies controlvoltage signals to the color switch 22, as generally indicated by thecoupling of the controller 26 and the color switch. The type of controlsignal that is sent depends on the specific implementation of the colorswitch 22, as will be appreciated by those skilled in the art. Incertain embodiments, the color switch 22 may be a rotating filter, inwhich case, the control signals supplied by the controller 26 will besignals that control the rotation of a motor drive (not shown) for thecolor switch 22. On the other hand, in certain other embodiments, thecolor switch 22 may be an LCD-based device. In this case, the controlsignals supplied by the controller 26 to the color switch 22 will besignals that control states of the color switch 22 that pass or filterparticular color components of the light 23, as will be described below.Alternatively, a different controller (not shown) could be used to sendcontrol voltage signals to the color switch 22, rather than thecontroller 26.

[0037]FIG. 2 illustrates a high resolution color image system 100 inaccordance with an exemplary embodiment of the invention. In the system100, the color switch 22 in FIG. 1 may be implemented as a color switch22′ for two-color operation, and include two different dichroic filtersor dichroic mirrors 22′A and 22′B, one for each color. In certain otherembodiments, the color switch 22′ can be implemented as a multifacetedreflector, such as a polygonal reflector, with each consecutivereflective surface alternating between filters like the filter 22′A andfilters like the filter 22′B. A multifaceted reflector may beadvantageous in that the rotational rate for driving the reflector, forexample, by a motor drive, can be reduced while still providing a rapidalternation between the filters, depending, among other things, on thenumber of facets.

[0038] In the embodiment shown in FIG. 2, the dichroic filters 22′A and22′B are mounted separately for rotation (generally indicated by arrows27) about an axis perpendicular to the direction of the incident light(i.e., an axis generally perpendicular to the drawing page in FIG. 2)and may be rotationally driven by a motor (not shown) via suitablecoupling. The controller 26, in this embodiment, provides controlvoltage signals to the drive motor, as discussed above. Moreover, thereflector 28, as described above, may be used in the system 100. Eachdichroic filter 22A′ and 22B′ is oriented by periodic rotation aboutthis axis for receiving the green and blue light 23, and only passesthem sequentially and alternately in a spectral sweep across agreen-blue LCD 16′ (e.g., the same type of LCD or imager as the LCD 16).The light 23 has sequential green and blue images imparted thereon andis reflected from the LCD 16′ back to the color switch 22′ to betransmitted as the light 25 to the display 24. The alternatingpresentation of the filters 22′A and 22′B to the green and blue light 23represents effective rotational “states” of the color switch 22′, onefor each of the two colors.

[0039] Examples of other suitable color filters, switches, or lightvalves that may be used for the color switch 22 include the systemsdisclosed in concurrently filed, co-owned U.S. pat. application Ser. No.______, filed ______, by Austin Huang and Richard M. Knox, entitled“Separating White Light Into Polarized, Colored Light,” which isincorporated by reference herein in its entirety. Such systems employ anelectro-optic shutter or filter, and will be described below in greaterdetail. The electro-optic shutter has states or modes and can switchbetween them in response to control voltage signals received from acontroller, as discussed above. In one state, the electro-optic shuttersubstantially passes a set of colors of light, for example, two colorssuch as red and green. In another state, the electro-optic shuttersubstantially passes another set of colors that includes one colorcommon to the original set and another color not common to those in theoriginal set, such as red and blue. Thus, each state passessubstantially at least one (e.g., primary) color that is not passed bythe other state. In either of the different states, the electro-opticshutter substantially excludes from passage one or more different colorsthat are not common to the different states.

[0040] To illustrate how an electro-optic shutter, such as thatdescribed above, may be advantageously employed as a color switch forperforming the same function as the color switch 22 in the system 10 inFIG. 1, reference is made to FIG. 3. In FIG. 3, in a high resolutioncolor image system 200, in accordance with an embodiment of theinvention, an electro-optic color switch (e.g., shutter 22″), is usedinstead of the color switch 22. The shutter 22″ is located between thelight source 12 and the reflecting polarizer 18. In certain embodiments,the shutter 22″ is mounted at an end of a tapered light pipe (TLP), atype of light guide employing total internal reflection (TIR). The otherend of the TLP would be positioned at or near the output of the lightsource 12 to couple light into the TLP.

[0041] The shutter 22″ may be operated so that two color (e.g., greenand blue) components alternately pass through the shutter 22″, whilealways passing a third (e.g., red) component of the input light 13(e.g., as might be useful in a field sequential color system, as will beappreciated by those skilled in the art). The shutter 22″ switchesbetween its (e.g., two) states or modes under control of the controller26 and is adapted to support a refresh rate of at least 60 Hz.Alternatively, a separate controller (not shown) could be used. Incertain embodiments, a linear polarizer 29, positioned between the lightsource 12 and the shutter 22″, is used to polarize the light 13, as willbe described below in more detail. Use of the linear polarizer 29 couldallow the reflecting polarizer 18 to be a mirror instead of a reflectingpolarizer. If the reflecting polarizer 18 were not a mirror, anintrinsic optical axis of the polarizer 29 would have to beappropriately aligned with the intrinsic optical axis of the reflectingpolarizer 18 for operation of the system 10 (or other similar systemsdisclosed herein). The linear polarizer 29 is included unless the lightsource 12 itself produces linearly polarized light for the light 13 orlinear polarized light is produced by another means. Such other meanscould be a polarizing (e.g., MOF) reflector situated between the lightsource 12 and the color switch 22″ that allows a preferred polarizationto pass while reflecting a non-preferred polarization back to the lightsource 12. This reflecting polarizer could replace the optionalreflector 28 as a way to avoid first surface reflection effects towardthe display 24 if light reflected back from the reflector 28 exhibitedsuch effects.

[0042] The shutter 22″ alternates passage of green and blue componentsof the light 13 by temporally switching their transmission, so that in afirst time period, substantially all the red and green wavelengths inthe incident portion of light 13 are transmitted to an outgoing portionof the light 13A that is reflected by the reflecting polarizer 18 andsplit by the one-color filter 20 into the light 15 and 23, as before.During the first time period, shown schematically in FIG. 4, the shutter22″ does not substantially transmit the blue component light in theincident portion of the light 13. In a second time period, the shutter22″ has switched and transmits substantially all the red and bluewavelengths in the incident portion of the light 13 to the outgoingportion of the light 13A. During the second time period, the shutter 22″does not substantially transmit the green wavelengths in the incidentlight 13 to the outgoing portion of the light 13A.

[0043] The electro-optic shutter 22″ may be adapted to temporally switchthe two component (e.g., green and blue) colors of the light 13 toapproximately equalize, increase, or decrease their time averageintensities relative to the third nonswitched (e.g., red) color in thelight 13. This may be desirable, if, for example, the light source 12 iscolor-deficient in the third color component. In other words, the thirdcolor component has an average intensity, as output from the lightsource 12, that is less than the average intensities of the other twocolor components, as discussed in the aforementioned concurrently filedU.S. pat. application Ser. No. ______. This may be understood byreferring to FIG. 4, which illustrates the color switching action of theelectro-optic shutter 22″. By adjusting the length of first and secondtime periods that the green and blue light components of the light 13pass through the shutter 22″, for example, using the controller 26,their average intensities may be adjusted relative to the intensity ofthe red component. This may be accomplished by time or pulse widthmodulation of the shutter 22″, and is possible because the red lightcomponent of the light 13 is arranged to be continuously available, asshown in FIG. 4.

[0044] The length of the switched first and second time periods in FIG.4 are limited, to a certain extent, by the switching frequenciesobtainable with the electro-optic shutter 22″. In embodiments of theelectro-optic shutter 22″ that employ FLCs or nematic liquid crystals,the maximum switching frequencies are about 100 KHz (as discussed above)and about 100 MHz, respectively. The present invention is intended toinclude embodiments of the shutter 22″ that employ any optically activeliquid crystal.

[0045] Referring to FIGS. 5A-5D, an implementation of the shutter 22″ indifferent states is illustrated as an electro-optic shutter 30 inaccordance with another exemplary embodiment of the invention. From theincident portion of the light 13 from the light source 12 (shown as red,green, and blue for illustrative purposes in FIGS. 5A-5D), theelectro-optic shutter 30 produces the outgoing light 13A. The incominglight 13 passes through the linear polarizer 29 (not shown in FIGS.5A-5D) that is positioned between the light source 12 and the shutter30. Therefore, the light 13 is assumed to already be linearly polarized,as illustrated by arrows in FIGS. 5A-5D. The linearly polarized light 13enters the electro-optic shutter 30, which includes liquid crystallayers 94, 96 and first and second color-selective polarizes 86, 88. Thelight 13 passes through the liquid crystal layer 94, then through thefirst and second color-selective polarizers 86, 88, and finally throughthe liquid crystal layer 96. The light 13 is subsequently output as thecolored and polarized light 13A.

[0046] The electro-optic shutter 30 may be constructed as asandwich-type structure (not shown in detail in FIGS. 5A-5D) containingthe liquid crystal layers 94, 96, as discussed in the aforementionedconcurrently filed U.S. pat. application Ser. No. ______. In someembodiments, the liquid crystal layers 94, 96 are constructed withliquid crystals selected from the group consisting of ferroelectricliquid crystals and twisted nematic liquid crystals. Transparentelectrodes (e.g., constructed of indium tin oxide or ITO) may be appliedon both sides of glass plates (not shown) that surround each of theliquid crystal layers 94, 96 to facilitate application of voltagesacross the liquid crystal layers 94, 96. The glass plates providestructural rigidity to the shutter 30, and pairs of the transparentelectrodes are layered on each pair of these glass plates in order toprovide an electric field across the liquid crystal layers 94, 96 forchanging their optical properties under control of the controller 26.Additional polymer layers may also be applied between the electrodelayers and the sides of the liquid crystal layers 94, 96 toward whichthey face. The additional polymer layers may be constructed ofpolyimide.

[0047] The shutter 30 may be electrically operated by the controller 26(FIG. 3), as discussed above, to switch or toggle the colors in theoutgoing light 13A by applying the voltages from a source (not shown)across one or both of the liquid crystal layers 94, 96. The controller26 may apply a voltage to one, both or neither of the liquid crystallayers 94, 96 at any particular time during operation of the shutter 30.Electrical operation in this manner changes the optical properties ofthe shutter 30 by either applying or not applying these voltages.

[0048] The two liquid crystal layers 94, 96 and the color-selectivepolarizers 86, 88 are optical devices that can exhibit birefringence.The layers 94, 96 have two voltage-controlled states: a “FIRST” or “OFF”state and a “SECOND” or “ON” state. In the SECOND state, each of theliquid crystal layers 94, 96 is birefringent and behaves approximatelyas a {fraction (1/2)}-wave phase retarder, i.e., a layer that rotatesthe polarization of incoming light of a selected wavelength or colorrange by approximately 90°. Because the incoming light 13 containsseveral selected wavelengths or color ranges, e.g., red, green, and bluewavelength ranges, the first and second liquid crystal layers 94, 96only approximately rotate the polarization of each of the selectedwavelength ranges by 90° when in the SECOND state. The precise rotationhas a small wavelength dependence. In the FIRST state, the liquidcrystal layers 94, 96 are not birefringent, i.e., in the FIRST state,the liquid crystal layers 94, 96 do not (or minimally) rotate thepolarization of the incoming light 13.

[0049] The first and second color-selective polarizers 86, 88 may beconstructed of cholesteric liquid crystal, polycarbonate, or any othersuitable reusable type of retarder material available from Colorlink,Inc. and other manufacturers. The polarizers 86, 88 may be similar topassive devices described in U.S. Pat. No. 4,425,028, issued to Gagnonand Carson, entitled “High Efficiency Optical Tank For Three ColorLiquid Crystal Light Valve Image Projection With Color SelectivePrepolarization And Single Projection Lens” and U.S. Pat. No. 4,544,237,issued to Gagnon, entitled “High Efficiency Optical Tank For Two-ColorLiquid Light Valve Image Projection With Color SelectivePrepolarization.” The polarizers 86, 88 may also be similar to passivedevices described in PCT application No. PCT/US96/07527, InternationalPublication No. WO 96/37806, filed May 23, 1996, by Gary D. Sharp,entitled “Color Polarizers,” published Nov. 28, 1996. U.S. Pat. Nos.4,425,028 and 4,544,237, and published PCT application No.PCT/US96/07527 are incorporated by reference herein in their entirety.

[0050] The first and second color-selective polarizers 86, 88 transmitselected colors and polarizations of light. The first color-selectivepolarizer 86 transmits both polarizations of light in the red and greenwavelength range and transmits blue light that is polarizedperpendicularly to the polarizing direction of the polarizer 29, whichis generally in a direction perpendicular to the drawing sheet in FIG.3. An internal optical direction of the first color-selective polarizer86 is aligned with the polarization direction of the polarizer 29 tocorrelate the polarization selectivity of both devices. Similarly, thesecond color-selected polarizer 88 transmits both polarizations of lightin the red and blue wavelength ranges. The color-selective polarizer 88transmits the light in the green wavelength range that is polarizedparallel to the polarizing direction of the polarizer 29. An internaloptical direction of the second color-selective polarizer 88 is alignedwith the polarizing direction of the polarizer 29 to correlate thepolarization selectivity of both devices. The color-selective polarizers86, 88 reflect substantially all visible light of polarizations inwavelength ranges that are not transmitted.

[0051]FIG. 5A illustrates the electro-optic shutter 30 when the liquidcrystal layers 94, 96 are both in the FIRST state, i.e., no voltage isapplied. In an input region 32 the polarized red, green, and blue light13 enters the shutter 30 as polarized by the polarizer 29 shown in FIG.3. Because the first liquid crystal layer 94 is in the FIRST state, thelight 13 from the region 32 is transmitted to a second region 34 without(or with minimal) polarization rotation. The first color-selectivepolarizer 86 substantially transmits only red and green light having theinitial polarization of the light 13 in the input region 32, and a thirdregion 36 substantially receives only red and green light having theinitial polarization. The blue light in the initial polarization stateis substantially not transmitted by the color-selective polarizer 86.Because the second color-selective polarizer 88 substantially transmitsall colors of light having the initial polarization, a fourth region 38substantially receives only red and green light having the initialpolarization. The second liquid crystal layer 96, being in the FIRSTstate, does not substantially rotate (or minimally rotates) lightpolarization, and an output region 40 substantially receives theavailable red and green light, having the initial polarization of thelight 13 in the input region 32, as the output light 13A.

[0052]FIG. 5B illustrates the electro-optic shutter 30 when the liquidcrystal layers 94, 96 are both in the SECOND state. In the input region32, the linearly polarized incident red, green, and blue light 13 entersthe shutter 30. Because the first liquid crystal layer 94 is in theSECOND state, the polarization of all light entering into the secondregion 34 is rotated. The second region 34 substantially receives thered, green, and blue light with polarization rotated by about 90°, i.e.orthogonal to the incident polarization. The first color-selectivepolarizer 86 transmits substantially all the input light with apolarization rotated by about 90°. Therefore, all three red, green, andblue colors enter the third region 36 with substantially the 90° rotatedpolarization. The second color-selective polarizer 88 substantially onlytransmits red or blue light, having the approximately 90° rotatedpolarization, into the fourth region 38. Because the second liquidcrystal layer 96 is in the SECOND state, the second liquid crystal layer96 rotates the polarization of light incident thereon by approximately90°. Then, the output region 40 substantially receives the red and bluelight in the initial polarization state of the light 13 in the inputregion 32.

[0053]FIG. 5C illustrates the electro-optic shutter 30 when the liquidcrystal layer 94 is in the SECOND state and the second liquid crystallayer 96 is in the FIRST state. Linearly polarized red, green and bluelight 13 enters in the input region 32. Because the first liquid crystallayer 94 is in the SECOND state, the polarization of substantially allthe light incident thereon is rotated. Thus, the second region 34substantially receives light of all three colors with the polarizationrotated by about 90°. The first color-selective polarizer 86 transmitssubstantially all light having the rotated polarization. Thus, the thirdregion 36 substantially receives light of red, green, and blue colorspolarized substantially orthogonal to the polarization of the light inthe input region 32. The second color-selective polarizer 88 transmitssubstantially all red and blue light with the rotated polarization andreflects substantially all green light having the rotated polarization.Thus, the fourth region 38 substantially receives the red and blue lightwith the substantially rotated or orthogonal polarization. Because thesecond liquid crystal layer 96 is in the FIRST state, it does notsubstantially rotate the polarization of light incident thereon.Therefore, the output region 40 substantially receives red and bluelight having a polarization rotated by approximately 90° with respect tothe initial polarization of the light 13 in the input region 32.

[0054]FIG. 5D illustrates the electro-optic shutter 30 when the firstliquid crystal layer 94 is in the FIRST state and the second liquidcrystal layer 96 is in the SECOND state. Because the first liquidcrystal layer 94 is in the FIRST state, light is substantiallytransmitted by the first liquid crystal layer 94 from the input region32 without substantial polarization rotation. The second region 34substantially receives red, green, and blue light that has the initialpolarization of light 13 from the left region 32. The firstcolor-selective polarizer 86 substantially transmits all the red andgreen light with the initial polarization and reflects substantially allthe blue light with the initial polarization. The third region 36substantially receives the red and green light with the initialpolarization. The second color-selective polarizer 88 substantiallytransmits all the red and green light with the initial polarization tothe fourth region 38. Because the second liquid crystal layer 96 is inthe SECOND state, it substantially rotates the polarization of lightincident thereon by about 90°. Thus, the output region 40 substantiallyreceives the red and green light having a polarization substantiallyorthogonal to the initial polarization of the light 13 in the inputregion 32.

[0055] FIGS. 6A-6B illustrate another implementation of theelectro-optic shutter 22″ shown in FIG. 3 as an electro-optic shutter 50in accordance with another exemplary embodiment of the invention. Thisembodiment, when implemented as in FIG. 3, does not require the linearpolarizer 29, as discussed above. The electro-optic shutter 50 includesa color-selective layer 104, a liquid crystal layer 106, and a linearpolarizer 108. The electro-optic shutter 50 may be constructed usingglass plates, transparent electrodes, etc., as similarly described abovefor the electro-optic shutter 30, and as disclosed in the aforementionedconcurrently filed U.S. pat. application Ser. No. ______. The controller26 or another controller may be used to control the states of theelectro-optic shutter 50 (i.e., states of the liquid crystal layer 106),as similarly discussed above. In FIG. 6A, the liquid crystal layer 106is shown when it is in a FIRST or OFF state (similar to the FIRST statedescribed above). In an input region 52, the incoming unpolarized light13 of red, green, and blue colors enters the shutter 50. Thecolor-selective layer 104 substantially transmits both polarizations ofthe red light, a first polarization of the green light, and asubstantially orthogonal polarization of the blue light to a secondregion 54. The manufacture of the color-selective layer 104 from, forexample, cholesteric liquid crystal layers is known in the art. Thepolarization of green light transmitted by the layer 104, e.g., acholesteric layer, is parallel to an intrinsic optical axis of the layer104. Cholesteric layers with preselected wavelength ranges can beobtained from Rolic Ltd., Postfach 3255, Basel Switzerland CH-4002 andfrom other manufacturers. Materials used to construct thecolor-selective layer 104 are also available from Colorlink, Inc. Thematerial used to construct the color-selective layer may be similar tomaterials used to construct the color-selective polarizers 86, 88.Moreover, the color-selective layer may be similar to devices describedin the aforementioned U.S. Pat. Nos. 4,425,028 and 4,544,237, and inpublished PCT application No. PCT/US96/07527.

[0056] In FIG. 6A, because the liquid crystal layer 106 is in the FIRSTstate, it substantially transmits the light from the second region 54 toa third region 56 without a substantial polarization rotation. Thepolarizer 108 substantially transmits to an output region 58, the redlight of polarization parallel to an intrinsic optical axis of thepolarizer 108 and the green light of the first polarization discussedabove as the output light 13A. Appropriate alignment of the intrinsicoptical axis of the polarizer 108 with that of the reflecting polarizer18 will allow the red and green light 13A to be reflected from thereflecting polarizer 18 for operation of the system 200.

[0057]FIG. 6B illustrates the electro-optic shutter 50 when the liquidcrystal layer 106 is in a SECOND or ON state (similar to the SECONDstate described above). Again, the color-selective layer 104substantially transmits both polarizations of red light, the firstpolarization of green light, and a substantially orthogonal polarizationof blue light in the light 13 from the first region 52 to the secondregion 54. Because the liquid crystal layer 106 is in the SECOND state,it substantially rotates the polarization of light substantiallytransmitted from the second region 54 to the third region 56 by about90°. The third region 56 substantially receives the red light of bothpolarizations, the green light of the orthogonal polarization, and theblue light of the first polarization. Again, the polarizer 108substantially only transmits polarizations substantially parallel to itsintrinsic optical axis to the output region 58 as the light 13A.Therefore, the red and blue light are substantially transmitted to theoutput region 58 in response to the electro-optic shutter 50 being inthe SECOND state. Again, appropriate alignment of the intrinsic opticalaxis of the polarizer 108 with that of the reflecting polarizer 18 willallow the light 13A, now red and blue, to be reflected from thereflecting polarizer 18 for operation of the system 200.

[0058] In the FIRST state, the electro-optic shutter 50, as shown inFIG. 6A, substantially transmits red and green light, while in theSECOND state, the shutter 50, as shown in FIG. 6B, substantiallytransmits red and blue light. In both states, the incoming white orquasi-white light beam 13 is unpolarized and the outgoing light beam 13Ahas a linear polarization that does not depend on whether the shutter 50is in the SECOND or the FIRST state. However, in various embodiments,the electro-optic shutter 50 is designed to produce different colors inthe outgoing light 13A in response to being in the SECOND and FIRSTstates.

[0059] Thus, as will be appreciated by those of skill in the art, theswitch 22″ may be constructed using either of the electro-optic shutters30 or 50 shown in FIGS. 5A-5D and 6A-6B, respectively, to provide, forexample, continuous one-color (e.g., red) and sequential and alternatetwo-color (e.g., green and blue) light as output.

[0060] Further information on electro-optic devices, modulators,color-selective layers, and filters employing liquid crystal devices ordisplays, and their effects on light color and light polarization may befound in U.S. Pat. No. 5,686,931, entitled “Device for Displaying ColorsProduced By Controllable Cholesteric Color Filters,” issued toFiinfschilling et al. and in International Application Published UnderThe Patent Cooperation Treaty (PCT) No. PCT/US97/08290, filed May 14,1997, by Kristina M. Johnson and Gary D. Sharp, entitled “ColorSelective Light Modulators,” International Publication No. 97/43862,published Nov. 20, 1997. U.S. Pat. No. 5,686,931 and PCT application No.PCT/US97/08290 are incorporated by reference herein in their entirety.

[0061] The present invention is also related to projection displaysystems. Information on projection display systems, and the use ofpolarization in such systems, can be found in prior, co-owned U.S. pat.application Ser. No. 08/581,108, filed Dec. 29, 1995, entitled“Projecting Images” and the aforementioned U.S. pat. application Ser.No. 08/747,190, and in European Pat. application No. 96309443.8, EPO783133A1, filed Dec. 23, 1996, also entitled “Projecting Images,”published Jul. 9, 1997. The contents of U.S. pat. application Ser. No.08/581,108 and the published European Pat. application No. 96309443.8,EPO 783133A1 are incorporated by reference herein in their entirety.

[0062]FIG. 7 illustrates a rear projection system 250 similar to thesystems described in the aforementioned U.S. pat. application Ser. Nos.08/581,108 and 08/747,190, and European Pat. application No. 96309443.8,EPO 783133A1. The rear projection system 250 can advantageously employthe high resolution color image systems 10, 100, or 200 in accordancewith embodiments of the invention. The system 250 may be used as part ofor in a computer monitor or television display.

[0063] The display apparatus 200 includes an imager or image source 202(e.g., in the systems 10, 100, or 200 that produces the light 25). Theimager 202 may be included in image engines similar to those describedin U.S. pat. application Ser. No. 08/730,818, filed Oct. 17, 1996,entitled “Image Projection System Engine Assembly,” which isincorporated by reference herein its entirety. The imager 202 outputsimage light 254 (e.g., the light 25) in response to input signals, forexample, electronic, video, or other signals received from an antenna,cable, computer, or controller (e.g., from the controller 26 or anothersource). The image light 254 reflects off a lower mirror or reflector256 to a higher mirror or reflector 258. The light 254 is then reflectedby the upper mirror or reflector 258 and is directed to a screen 260.The screen 260 may be a diffusive screen or diffuser. The screen 260scatters the image light 254 as light 262, which a viewer 264 can see asforming an image at the screen 260 of the display system 250.

[0064]FIGS. 8 and 9 illustrate another rear projection video displaysystem 300 similar to systems described in the aforementioned U.S. pat.application Ser. Nos. 08/581,108 and 08/747,190, and European Pat.application No. 96309443.8, EPO 783133A1. The rear projection videodisplay system 300 can advantageously employ the high resolution colorimage systems 10, 100, or 200 in accordance with embodiments of theinvention. The system 300 may be used as part of or in a computermonitor or a television display. The system 300 is also similar tosystems described in prior, co-owned U.S. pat. application Ser. No.08/880,178, filed Jun. 20, 1997, by Richard M. Knox, also entitled“Projecting Images” and in concurrently filed, co-owned U.S. pat.application Ser. No. ______, by Austin Huang and Richard M. Knox,entitled “Producing Colored Light Beams From White Light,” bothincorporated by reference herein in their entirety.

[0065]FIG. 9 is a blow-up view illustrating a portion 302 of the rearprojection video system 300 shown in FIG. 8. The system 300 includes areflecting linear polarizer 304 (which may be similar to the reflectingpolarizer 18) and an achromatic retarder 306 that form a “folded”optical train or “folded” optics 308 for projecting an image on adisplay screen 310 (e.g., the display screen 24, which may be a diffuseror diffusive screen). Optical “folding” enables the system 300 (and thesystem 150 in FIG. 7) to be shallow, i.e., to have a small depthfootprint (L in FIG. 7 and L′ in FIG. 8 for an apparent projectionlength), while still projecting a large image. A portion of image light314 (e.g., the light 25) from an image engine 316 (e.g., in the systems10, 100, or 200 that produces the light 25) reflects from the reflectingpolarizer 304 of the folded optical train 308 at one instance 320 in onepolarization state, and then passes through the achromatic retarder 306with polarization being shifted by approximately 45°. The portion of theimage light 314 then reflects from a reflector or mirror 318, passes asecond time through the achromatic retarder 306 with polarization beingshifted by another approximately 45°, and then passes through thereflecting polarizer 304 to the display screen 310 at another instance324. The screen 310 scatters the light 314 as light 325 which can beviewed by a viewer 326. The image light 314 being transmitted to thedisplay screen 310 through the intervening folded optical train 308 maybe viewed as an alternative to the image light 25 being transmitted tothe display 24 in FIGS. 1-3 in the systems 10, 100, and 200 describedabove. The total polarization shift of approximately 90° due to thedouble pass through the achromatic retarder 306 allows the portion ofthe image light to pass through the reflecting polarizer 304 to thedisplay screen 310 for display. The system 300 is suitably arranged forthe polarization direction of the image light 314 to be correlated tothe intrinsic optical axis of the reflecting polarizer 304 for thisoccurrence, as will be appreciated by those skilled in the art.

[0066] The image engine 316 may include an imager or image source 327(e.g., in the systems 10, 100, or 200 that produces the light 25) thatreceives electrical signals through an input cable 328 (from, e.g., thecontroller 26 or another source), and converts the signals into theprimary image beam 314. The types of electrical signals used to createthe primary image beam 314 may include television signals, such as thosereceived by an antenna or over cable lines and processed by a videoreceiver (not shown), and computer video signals generated by a computersystem (not shown). The image source 327 may also be included in anyconventional image projection engine, such as a liquid crystal display(LCD) projector.

[0067] A signal splitter (not shown) and a sound system 329 can also beincluded in the system 300. The signal splitter divides the electricalsignals into, for example, a video signal and an audio signal, andprovides these signals to the image engine and the sound system 329,respectively. The image engine converts the video signal into theprojected image light 314. The audio signal and the sound system 329 areoptional.

[0068] In addition to the image source 327, the image engine alsoincludes a light source 330 (e.g., the light source 12) that outputslight (e.g., the light 13 although not shown in FIG. 8). The imagesource 327 must produce polarized light as its output (e.g., using thepolarizer 29 or the reflecting polarizer 18, or using a polarizer in thelight source 330). A wide variety of other types of video systems employpolarization in image formation. The light source 330 generates lightincident on the image source 327 to create the image light 314. Examplesof light sources that may be used as the light source 330 include thosedescribed in the aforementioned U.S. Pat. No. 5,404,076 and U.S. pat.application Ser. Nos. 08/747,190 and 08/771,326.

[0069] The particular embodiments disclosed above are illustrative only,as the invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. For example, it will be appreciated bythose skilled in the art that any of the systems 10, 100, 200, or 300could be advantageously employed and support a field sequential colorprojection or display system. Furthermore, no limitations are intendedto the details of construction or design herein shown, other than asdescribed in the claims below. It is therefore evident that theparticular embodiments disclosed above may be altered or modified andall such variations are considered within the scope and spirit of theinvention. Accordingly, the protection sought herein is as set forth inthe claims below.

What is claimed is:
 1. A system used in image projection, comprising: afirst filter for receiving light, for directing a first portion of lightto a first image source for first portion operation, and for passing asecond portion of the light to a second image source for second portionoperation; and an electro-optic color switch for alternately allowingcomponents of the second portion to be passed to the second imagesource.
 2. The system of claim 1, wherein the first portion operationcomprises one-color operation and the second portion operation comprisestwo-color operation.
 3. The system of claim 1, wherein a maximumswitching rate of the second image source is sufficient for providingtwo-color sequential operation, but insufficient for providingthree-color sequential operation for a given color depth.
 4. The systemof claim 1, further comprising a second filter for passing onepolarization of the first and second portions of the light, and forreflecting one other polarization of the first and second portions ofthe light to the first filter.
 5. The system of claim 4, wherein thesecond filter comprises a non-angle-specific polarizing and reflectingmaterial.
 6. The system of claim 4, wherein the second filter comprisesmultilayer optical film.
 7. The system of claim 4, further comprising areflector for receiving light from the second filter and for reflectinglight back through the second filter.
 8. The system of claim 7, whereinthe reflector comprises multilayer optical film.
 9. The system of claim7, wherein the reflector is adapted to reflect the one polarization ofthe first and second portions of the light.
 10. The system of claim 7,wherein the reflector is adapted to help reduce possible detrimentalthermal effects.
 11. The system of claim 7, further comprising a lightsource adapted to provide the light received by the first filter,wherein the reflector is adapted to reflect the one polarization of thefirst and second portions of the light back to the light source foroptical pumping.
 12. The system of claim 1, wherein the first and secondimage sources are adapted to shift light polarization for returning apolarization-shifted image.
 13. The system of claim 1, wherein theelectro-optic color switch comprises a control device for switchingbetween the components of the second portion of the light at a ratebetween approximately 200 and approximately 400 per second.
 14. Theimage projection system of claim 13, wherein the electro-optic colorswitch, in a first state, is adapted to pass a first component of thesecond portion of the light, and, in a second state, to pass a secondcomponent of the second portion of the light.
 15. The system of claim14, wherein the control device is operatively coupled for alternatingthe color switch between the first and second states.
 16. The system ofclaim 14, wherein the first and second states comprise electro-opticstates of the electro-optic color switch.
 17. The system of claim 14,wherein the first and second states comprise rotational states of theelectro-optic color switch.
 18. The system of claim 1, wherein theelectro-optic color switch comprises an electro-optic shutter.
 19. Thesystem of claim 1, wherein the electro-optic color switch comprisesalternating dichroic reflectors.
 20. The system of claim 1, furthercomprising a controller for receiving video data and for controlling thefirst and second image sources in response to the video data.
 21. Thesystem of claim 1, wherein the electro-optic color switch is adapted toreceive the second portion of the light from the first filter.
 22. Thesystem of claim 1, wherein the first filter is adapted to receive thefirst and second portions of the light from the electro-optic colorswitch.
 23. The system of claim 1, wherein the first filter and theelectro-optic color switch are comprised in part of a field sequentialcolor system.
 24. An image engine, comprising: a first image sourcehaving a first response time for color operation at a color depth, thefirst response time being greater than that required for operation ofthe first image source at some other color depth; a second image sourcehaving a second response time for color operation at the color depth,the second response time being greater than that required for operationof the second image source at the other color depth; a color filter fordirecting colored light to the first image source and for directingother colored light to the second image source; and an electro-opticcolor switch for alternately allowing components of the other coloredlight to be directed to the second image source.
 25. The image engine ofclaim 24, wherein the colored light comprises red and the other coloredlight comprises green and blue.
 26. The image engine of claim 24,wherein the colored light and the other colored light comprise red,green, and blue light.
 27. The image engine of claim 24, wherein theelectro-optic color switch is adapted
 28. The image engine of claim 24,wherein the electro-optic color switch is adapted to filter the othercolored light.
 29. The image engine of claim 24, wherein theelectro-optic color switch is adapted to alternate between thecomponents of the other colored light under control of a controllerdevice.
 30. The image engine of claim 24, wherein the electro-opticcolor switch comprises an electro-optic shutter.
 31. The image engine ofclaim 24, wherein the electro-optic color switch comprises alternatingdichroic reflectors.
 32. The image engine of claim 24, wherein theelectro-optic color switch is disposed between the color filter and thesecond image source.
 33. The image engine of claim 24, wherein the colorfilter is disposed between the electro-optic color switch and both thefirst and second image sources.
 34. The image engine of claim 24,wherein the first and second image sources comprise liquid crystaldisplays.
 35. The image engine of claim 24, wherein the first and secondimage sources comprise ferroelectric liquid crystal displays.
 36. Theimage engine of claim 24, wherein the operation of the first and secondimage sources at the other color depth comprises three color operation.37. The image engine of claim 24, wherein the color depth comprises6-bit color depth and the other color depth comprises 8-bit color depth.38. The image engine of claim 24, wherein the other color depthcomprises a greater color depth than the color depth.
 39. The imageengine of claim 24, wherein the first and second image sources comprisefirst and second pixel arrays, respectively.
 40. The image engine ofclaim 24, wherein the first and second image sources, the color filter,and the electro-optic color switch are comprised in part of a fieldsequential color system.
 41. A method used in projecting an image, themethod comprising: directing a first colored portion of light from alight source to a first color imager; returning a colored image from thefirst imager; directing a second colored portion of the light from thelight source to a color-operative filter; alternately passing componentsof the second colored portion through the color-operative filter to asecond imager having a color switch rate insufficient for three-coloroperation at a given color depth; alternately returning other coloredimages from the second imager corresponding to the components of thesecond colored portion; and combining the colored image and the othercolored images as image light.
 42. The method of claim 41, furthercomprising projecting the image light onto a display.
 43. The method ofclaim 41, wherein directing the second colored portion comprisesdirecting the second colored portion to a two-color-operative filter.44. The method of claim 41, further comprising passing the first coloredportion through the color-operative filter to the first imager.
 45. Themethod of claim 41, wherein alternatively passing components comprisesalternatively passing light comprising two primary colors.
 46. Themethod of claim 41, wherein alternatively passing components comprisesalternately green and blue light through the color-operative filter tothe second imager, and wherein directing the first colored portioncomprises directing red light to the first imager.
 47. The method ofclaim 41, further comprising passing the first colored portion throughthe color-operative filter to the first imager.
 48. The method of claim41, further comprising reflecting the first colored portion from acolor-selective filter to the first imager and passing the components ofthe second colored portion through the color-selective filter to thecolor-operative filter.
 49. The method of claim 41, whereinsubstantially only the components of the second colored portion passthrough the color-operative filter.
 50. An image projection system,comprising: an image engine comprising: a light source, a first filteradapted to receive light from the light source for directing a firstportion of the light to a first image source for first portionoperation, and for passing a second portion of the light to a secondimage source for second portion operation, and an electronic colorswitch adapted to alternatively allow components of the second portionto be passed to the second image source; and a folded optical trainadapted to receive image light from the first and second image sources.51. The image projection system of claim 50, further comprising adisplay screen adapted to receive the image light.
 52. The imageprojection system of claim 50, wherein the folded optical traincomprises an achromatic retader adapted to retard a polarization of theimage light.
 53. The image projection system of claim 50, wherein thefolded optical train comprises a display screen adapted to receive theimage light.
 54. The image projection system of claim 50, wherein theimage engine and the folded optical train are comprised in part of acomputer monitor.
 55. The image projection system of claim 50, whereinthe folded optical train comprises a linearly reflecting polarizer. 56.The image projection system of claim 50, wherein the first and secondimage sources comprise liquid crystal displays.
 57. The image projectionsystem of claim 50, wherein the first and second image sources compriseferroelectric liquid crystal displays.
 58. The image projection systemof claim 50, wherein the electronic color switch comprises alternatingdichroic filters.
 59. The image projection system of claim 50, whereinthe electronic color switch comprises an electro-optic shutter.
 60. Theimage projection system of claim 50, wherein the image engine and thefolded optical train are comprised in part of a field sequential colorsystem.